This disclosure relates generally to the construction of subterranean wells. More particularly, this disclosure relates to methods and apparatus for centering a casing within a well, particularly an oil or gas well. Still more particularly, this disclosure relates to methods and apparatus for centering a casing within an oil and gas well such that cement can be evenly distributed within the annulus disposed between the casing to enable zonal isolation (i.e. no fluid or gas migration).
A well is a subterranean boring from the Earth's surface that is designed to find and acquire liquids or gases. Wells for acquiring oil are termed “oil wells”. A well that is designed to produce mainly gas is called a “gas well”. Typically, wells are created by drilling a bore, typically 5 inches to 40 inches (12 cm to 1 meter) in diameter, into the earth with a drilling rig that rotates a drill string with an attached bit. After the hole is drilled, sections of steel pipe, commonly referred to as “casings” and which are slightly smaller in diameter than the borehole, are dropped “downhole” into the bore for obtaining the sought after liquid or gas.
The difference between the diameter of the wellbore and the outer diameter of the casing results in an annular space therebetween. When completing oil and gas wells, it is desirable to seal the annular space with cement. The cement is pumped into the annular space, replacing the drilling mud therein. Once the annular space is filled with cement, the cement is allowed to harden to seal the well. To properly seal the well, the casing is preferably positioned so that it is in the middle or center of the wellbore such that the annular space has a constant or substantially constant radial width moving circumferentially about the casing. The casing and cement provide structural integrity to the newly drilled wellbore and provide isolation of high pressure zones. Thus, centralizing a casing inside the annular space is facilitates a reliable seal, and thus good zonal isolation. With the advent of deeper wells and horizontal drilling, centralizing the casing has become more important, yet more difficult to accomplish.
A traditional method to centralize a casing is to attach centralizers to the casing prior to its insertion into the annular space. Most traditional centralizers have tabs, wings or bows that exert force against the inside of the wellbore to keep the casing somewhat centralized. The centralizers are commonly secured at intervals along a casing string to radially offset the casing string from the sidewall of a borehole in which the casing string is positioned. Centralizers center the casing string within the borehole to provide a generally continuous annulus between the casing string and the sidewall of the borehole. This positioning of the casing string within a borehole promotes uniform and continuous distribution of cement slurry around the casing string. Uniform cement slurry distribution results in a cement liner that reinforces the casing string, isolates the casing from corrosive formation fluids, prevents unwanted fluid flow between penetrated geologic formations, and provides axial strength. In general, 100% standoff is achieved once the casing is radially centralized within the borehole or the annulus with the previous casing.
A bow-spring centralizer is the most common type of centralizer. It employs flexible bow-springs to provide offset between the casing and wellbore sidewall. Bow-spring centralizers typically include a pair of axially-spaced and generally aligned circular collars that are coupled by multiple bow-springs. The bow-springs expand outwardly from the collars to engage the borehole sidewall to center a pipe received axially through the collars. Configured in this manner, the bow-springs provide stand-off from the borehole, and flex inwardly as they encounter borehole obstructions, such as tight spots or protrusions into the borehole, as the casing string is installed into the borehole. Elasticity allows the bow-springs to spring back to substantially their original shape after passing an obstruction to maintain the desired stand-off between the casing string and the borehole. Examples of such bow springs are disclosed in U.S. Pat. No. 4,545,436 and Great Britain Patent No. 2242457 which both disclose casing centralizers having a plurality of bows springs which are connected to first and second collars. The collars surround the well casing, and one or both of the collars slide longitudinally upon the pipe when the bow spring is deformed upon engaging the well bore sidewall.
The use of bow-spring centralizers presents a number of disadvantages and their installation can be problematic. To achieve the desired centralization, bow centralizers are designed so that, prior to installation. the bow-springs extend beyond the inside diameter (“ID”) of the wellbore. The larger diameter of said bow-springs requires them to be retracted from the force of pushing it down inside the casing or wellbore. This causes kinetic friction when slid down the hole (requiring running force) and also static friction when engaging restrictions or obstructions (requiring starting force). This friction can sometimes prevent the casing from getting to the desired depth. Further, the radial configuration of the bow-springs causes the spring force of one bow-spring to be counteracted by the bow-springs on the opposite side of the casing. This results in a restoring force that diminishes as the casing approaches center, thereby making better centralization require greater and greater spring forces. Furthermore, increased spring forces also increases running and starting resistance. Therefore, a balance is sought between the needed forces to sufficiently centralize the casing and the increased resistance that these spring forces create. A further disadvantage of the bow spring centralizers can arise in wells where the open hole is under-reamed (i.e. the hole is larger than the previous casing shoe). In such cases, centralization is particularly difficult as the bows have to collapse through the restriction and expand sufficiently to centralize the casing in larger open hole.
Yet another disadvantage of bow spring centralizers is that the bow springs obstruct the pumping of cement downhole. After being positioned downhole, the bow springs project radially outward from the casing like spokes to engage the well bore's cylindrical wall. These bow springs can block the proper downward flow of the cement slurry or can create voids in the annular cement structure.
Various attempts have been made to develop centralizers that overcome some of these problems. U.S. Pat. No. 6,871,706 discloses a centralizer that requires the bending of a retaining portion of the collar material into a plurality of aligned openings, each to receive one end of each bow-spring. This requires that the coupling operation be performed in a manufacturing facility using a press. The collars of the centralizer are cut with a large recess adjacent to each set of aligned openings to accommodate passage of a bow-spring that is secured to the interior wall of the collar. Unfortunately, the recess substantially decreases the mechanical integrity of the collar due to the removal of a large portion of the collar wall to accommodate the bow-springs.
U.S. Patent Publication 20120279725 and U.S. Pat. No. 7,857,063 describe centralizers that have a minimal radial expansion prior and during the casing's transportation downhole. Only after the casing is in place are the centralizer tabs expanded radially outward. This reduces the amount of friction that the casing string encounters as it is dropped downhole. Furthermore, the tabs extend laterally relative to the pipe's central axis in a manner that minimizes the obstruction to the flow of cement as it poured downhole. Unfortunately, these centralizers are not suitable for traditional metal well casings that provide minimal radial expansion. Instead, the centralizers are useful only for centralizing tubular members capable of substantial expansion so as to force the centralizer tabs to engage the borehole wall.
Thus, there is a significant need for an improved casing centralizer that provides reduced friction as the centralizer is transported downhole.
There is also a need for an improved casing centralizer that provides increased centralizing force for maintaining a casing in the center of a well bore.
Still there is an additional need for an improved casing centralizer that provides minimal impedance to the flow of cement as cement is pumped downhole in the annular space between the casing string and the well bore wall.
Advantageously, the improved centralizer would provide reduced manufacturing and installation costs, and provide an improved ease of running the casing string downhole into the well bore.